1. Field of the Invention
This invention relates to a thermoplastic dental impression tray that may be shaped to conform to a patient's dental arch.
2. Description of the Related Art
Dental impression trays are used to hold impression material for making a model of a patient's oral cavity so that a crown, bridge, denture, restoration or the like can be made. To make the model, a quantity of impression material is placed in an open trough or channel of the tray, and the tray is then pressed onto the dental arch to make a female cast of the desired area of interest. The cured impression is then used to form a male model which replicates the selected area of the patient's arch.
The accuracy of fit of a restoration is due, in large part, on the dimensional accuracy of the impression that is taken from the patient's arch. Consequently, it is important that the impression is not distorted when the tray is removed from the mouth.
Recently, there has been increased interest in the use of thermoplastic impression trays which may be heated and then shaped to closely conform to the patient's dental anatomy. Since dimensions of the dental arch may vary widely from patient to patient, such trays may be molded when heated to adjust the height and width of the channel of the tray to accommodate the selected area of the patient's arch. In this manner, the tray may be shaped to conform to the arch so that an accurate impression may be made, while enabling the relatively expensive impression material to fully surround the selected area of the arch without wastage. Examples of such moldable impression trays are described in U.S. Pat. Nos. 4,227,877, 4,361,528 and 4,657,509.
Often, a considerable force must be used to disengage the cured impression material from the arch. The impression material closely conforms to the arch and a vacuum may be formed between the impression material and the arch as the tray is pulled, rendering disengagement difficult. In addition, the teeth and adjacent recesses often form undercuts in the impression material, such that the impression must be spread apart during disengagement of the impression from the arch.
Typically, the dentist will separate posterior portions of the cured impression from the arch before separating anterior portions of the impression. The channel of the tray normally has an open posterior end, and it is thus easier to release the vacuum formed between the impression material and the arch in areas adjacent the open end of the channel. Once the vacuum is released, it is somewhat easier to pull remaining portions of the impression including anterior portions from corresponding regions of the arch.
Conventionally, dentists purchase flat sheets of thermoplastic material and form the sheets in their offices to the shape of an impression tray. In some cases, handles are molded to the front of the tray to aid in delivery of the tray with the impression material to the oral cavity. The handle also enables the dentist to exert leverage to release posterior regions of the tray from the arch. However, such handles hinder shaping of the thermoplastic material when softened in anterior regions of the tray where the latter is joined to the handle.
Thermoplastic impression trays without handles are normally removed from the arch by placing the fingers over a buccal-gingival edge of the channel in posterior regions of the tray, and exerting a force in an occlusal direction to release posterior portions of the impression from corresponding posterior regions of the arch. However, such practice may spread apart the walls of the channel and permanently distort the impression since considerable force must often be used to release the vacuum. In addition, uncured impression material often is forced over the gingival edges of the channel as the tray is placed onto the arch, thus increasing the difficulty of gripping the buccal-gingival edge of the channel with the fingers after the impression material has cured.